1. Field of the Invention
The present invention relates to a method and an apparatus for extruding a powdered material to form a pellet, and in particular to continuously compact the powdered material into an elongated body such as a rod, a pole, a tube and the like.
2. Description of the Prior Art
In the field of powder metallurgy, a powdered material is pressed into a pellet with a desired shape before sintering, and then a standard compression molding method, such as that disclosed in U.S. Pat. No. 5,156,854, is used for compacting the powdered material. In the compression molding method, a mold with a molding cavity and upper and lower punches are used, in which the lower punch is fitted in the bottom of the molding cavity before a powdered material is poured into the molding cavity. The powdered material is then compressed by applying the upper punch to the cavity. However, if an elongated pellet is desired, thereby requiring the molding cavity to have a large longitudinal depth in comparison with the area of the surface pressed by the punch, it is difficult to uniformly press all the powdered material in the molding cavity. Consequently, the use of this method is limited to manufacturing a pellet which has a short length relative to the area of the pressed surface, and it is impossible to form a continuous pellet by this method. Therefore, when a longitudinal pellet is the goal, an extrusion method has been conventionally used.
In the conventional extrusion method, the powdered material is ordinarily mixed with a binder, such as wax and the like for imparting moldability to the powdered material, and converted into a paste; the powdered material alone would easily fall out of the die cavity due to its flowability. The obtained paste is then put into the container of the extruder, in which an extrusion operation is performed in a manner similar to that for an injection molding machine, and extruded through a cavity to form an extrudate, which is sintered thereafter to obtain a sintered product. In most cases, however, it is necessary to subject the extrudate to an aftertreatment to remove the binder from the extrudate.
Moreover, in the case of employing a hot extrusion technique, the type of raw powdered material and the operating temperature must be carefully selected to avoid oxidation of the powdered material. However, if such a temperature selection is difficult and the use of a material which is expected to be oxidized at a high temperature is unavoidable, an encapsulation method must be employed. In the encapsulation method, the raw material is enclosed in a sheath under vacuum and then subjected to hot extrusion. This treatment prevents the raw material from being oxidized. However, except for some special cases, the extrudate needs to be further treated to remove the sheath by chemical dissolution, etc.
As described above, for implementing the conventional extrusion method, a variety of additional and complicated steps (i.e. the paste-forming step, the wax-removing step, etc.) are necessary before or after the essential extrusion process. Accordingly, the conventional extrusion method, particularly the hot extrusion method, has adverse effects on manufacturing costs. Consequently, the technical field in which the extrusion method can be employed is at present limited to special cases in which the manufactured pellet has a high value added for economic reasons and in which the pellet manufactured by using the extrusion method can be imparted a particular property that cannot be obtained by the ordinary powder metallurgical method. For this reason, the extrusion method has not been generally utilized for manufacturing ordinary machine parts, in spite of the prominent characteristics present in that method.